A fuel cell is designed to extract the energy generated by the electrochemical reaction of a fuel and an oxidizer directly as electric energy, and is either being researched or is already practiced as either an electric power plant or a power source for aerospace devices, unmanned facilities on sea or shore, stationary or movable radios, automobiles or domestic appliances.
Fuel cells can be divided roughly into representative types such as molten salt electrolyte types of fuel cell, which operate at a high temperature (e.g., about 500.degree. to 700.degree. C.), phosphoric electrolyte types of fuel cell, which operate in the vicinity of 200.degree. C., or alkaline or acidic electrolyte types of fuel cell, which operate at room temperature or at a temperature of at most about 100.degree. C.
For the electrolyte of high-temperature fuel cells, carbonates such as K.sub.2 CO.sub.3, which are solid at room temperatures, have frequently been used.
Phosphoric fuel cells use phosphoric acid having a melting point of about 42.degree. C., or polyphosphoric acid, as disclosed in the specification of Japanese Pat. Laid-Open No. 5286/1982, both of which are solid or semi-solid at room temperatures.
The electrolyte frequently used in alkaline or acidic electrolyte types of fuel cell used at a temperature of at most 100.degree. C. is either an aqueous solution of caustic potash, caustic soda or lithium hydroxide, or dilute sulfuric acid. This is because aqueous solutions of these strong electrolytes are the most convenient for use because they have a high ionic conductivity at a low temperature.
Those strong electrolytic substances are so corrosive that they restrict the materials that can be used for composing the cell. This means that sufficient care must be taken to prevent those electrolytes from leaking from a cell. However, counter-measures against leakage of electrolyte in existing cells are not very simple, so that a variety of trials have been made in the prior art.
The basic problem is raised from the fact that the electrolyte is a liquid when the fuel or oxidizer is liquid. The phenomenon occurs that the electrolyte which should stay in the electrolyte chamber passes through a porous fuel or oxidizer electrode into the fuel or oxidizer chamber as a result of dilution due to the concentration gradient within the liquid fuel or oxidizer.
In a fuel cell using a liquid fuel, it is customary as a counter-measure to supply the fuel chamber with a fuel mixture (which is usually called an anodic electrolyte) which has been diluted with the electrolyte. The difference in the concentration of the electrolyte is so reduced by this that outflow of the electrolyte from the electrolyte chamber to the fuel chamber is reduced. Nevertheless, the dilution of the fuel with the electrolyte is a counter-measure which is unnecessary for the intrinsic functions of the cell, and the concentration of the fuel is also reduced accordingly so that more power is consumed for circulating the electrolyte than the fuel, with a resultant reduction in the energy efficiency. Moreover, the supply or circulation of the highly corrosive electrolyte together with the fuel is disadvantageous for the user, in addition to the limitations on the constructional materials.
Although there is an example in which an inorganic powder is mixed with the electrolyte to make a paste, this method has not succeeded in providing a basic counter-measure because the fuel or oxidizer electrode will essentially allow the electrolyte to pass therethrough.